This invention pertains generally to radio frequency receivers and particularly to receivers which are used in monopulse radar systems.
It is well known in the art that, in a conventional two or three channel monopulse receiver, imbalances between channels may cause unacceptably large errors to be experienced. To eliminate the causes of such errors in a multichannel monopulse receiver it is necessary that, in addition to providing properly designed and fabricated radio frequency comparators (sometimes referred to as "R.F. arithmetic units") which produce the required radio frequency sum and difference signals for the different channels in such types of receivers, the intermediate frequency sections of such receivers must be designed, fabricated and operated so that amplitude and phase imbalances are kept to a minimum. In particular, the I.F. amplifiers in the various channels must be so designed, fabricated and operated that, regardless of changes in signal level or environmental conditions, amplitude and phase imbalances between such amplifiers are reduced to a minimum. In the present state of the art, amplifiers exhibiting the required degree of stability are difficult to design and fabricate, making production of multi-channel monopulse receivers rather expensive and difficult.
It has been proposed to provide a monopulse receiver wherein the sum and difference signals out of R.F. arithmetic units are combined in such a manner that a single I.F. amplifier may be used for the sum and difference signals. Thus, in one such proposed approach, a plurality (say three) different local oscillators are provided, each producing one of three different I.F. signals to heterodyne with the sum and two difference signals out of an R.F. arithmetic unit in a monopulse receiver. With the frequencies of the local oscillators properly chosen, three I.F. signals within the bandpass of a single amplifier may be obtained so that the sum and two difference signals may be amplified simultaneously. The I.F. amplifier is designed to normalize the signals by hard limiting so that all signals out of such amplifier have the same amplitude. Further, if the frequency response of the I.F. amplifier is flat, all signals on passing through the I.F. amplifier are subject to the same phase shift. Thus, amplitude and phase imbalances are minimized. The three amplified signals out of the I.F. amplifier may be separated by appropriate filters and processed. While the single channel approach obviates much of the "balancing" difficulties experienced with the conventional two or three channel approach, other difficulties are experienced. For example, because the sum and difference signals passing through the I.F. amplifier differ only slightly in frequency, their complete separation by bandpass filters is difficult to achieve with the result that undesirable intermodulation may occur between the sum and difference signals. Further, such a "single channel" approach requires a plurality of local oscillators having different frequencies which must be precisely maintained during operation.